The invention relates to: a heat transfer sheet provided between a heat generating element such as a semiconductor device and a heat dissipating element such as a heat sink or a heat spreader to thereby transfer heat from the heat generating element to the heat dissipating element; a heat transfer structural body comprising the heat transfer sheet, the heat generating element and the heat dissipating element; and a manufacturing method of the heat transfer structural body.
Since considerably much heat is generated from a semiconductor device in recent years due to progress to higher speed in operation and higher complexity in integration, a necessity has arisen for efficient dissipation of generated heat to the outside. Conventionally, there has been generally employed a method of transferring heat generated in a semiconductor device to a heat sink, followed by dissipation thereof. In this method, a technique is adopted in which a heat transfer sheet made of a material with a thermal conductivity is bonded between a semiconductor device (heat generating element) and a heat sink (heat dissipating element) to reduce heat resistance in a bonding interface and to thereby improve a heat dissipation characteristic (for example, Japanese Patent Application Laid-Open 58-169912 (1983) and Japanese Patent Application Laid-Open 2003-51573(2003).
As a thermally conductive material used in heat transfer sheet, there has been well known a material obtained by dispersing an inorganic filler into a silicone resin, but with a problem of a low thermal conductivity. In addition, another thermally conductive material that has also been used is an electrically conductive paste obtained by dispersing a metal powder high in thermal conductivity such as Ag or Cu powder into an adhesive. However, since heat is transferred by point contacts between powders thereof, it is impossible to obtain a sufficiently high thermal conductivity.
On the other hand, a metal material such as solder has been used as still another thermally conductive material. A heat transfer sheet made of a metal material such as solder can exert an excellent thermal conductivity because of being a metal material, whereas heating is necessary in the bonding, which leads to a problem of degrading reliability of a bonded portion between a semiconductor device (heat generating element) and a heat sink (heat dissipating element) due to a thermal stress.
Temperature is not uniform in a recent semiconductor device with generation of much heat and a CPU core of a semiconductor device 31 is, as shown in FIG. 1, especially high in temperature as compared with the other portions, which arises a heat spot (high temperature region) 1a. A heat dissipation process is particularly important to deal with such a heat spot 1a, in which a high thermal conduction characteristic is required.